Symposium on Erin Pobjie’s Prohibited Force: Use of Force as Family Resemblances – A Cautionary Take

[Alejandro Chehtman is Dean and Professor of Law at Universidad Torcuato Di Tella.]

Prohibited Force (CUP, 2024) addresses a neglected issue in International Law, namely, the scope of the prohibition to use force. That this is a neglected issue might seem unexpected, given that this rule has been appropriately termed a “cardinal principle”, and one of the “cornerstones” of the international legal system. Yet the reason for this inattention is far from mysterious. Violations to this rule typically create political upheaval and are followed by intense diplomatic action and academic commentary. However, Erin Pobjie notes, cases at the “lower end of the intensity spectrum” present serious difficulties insofar state authorities are usually reluctant to position themselves where both the facts and the law are deeply uncertain. (In turn, most institutional and academic efforts have been concerned with the notion of an armed attack, a subset of all the violations of the prohibition to use force, given its momentous implications, most notably the fact that it unlocks an armed response in self-defense.) The book thereby develops a framework to better account for these latter violations to the prohibition to use force under international law. It is an important and timely contribution given that this type of situation, Pobjie indicates, has tended to “occur more frequently” and is typically of the type “over which uncertainty reigns” (p. 2). In effect, recent incidents of this type include the assassination of a former Russian spy in the UK in 2017, the use of maritime militia in disputed zones of the South China Sea, and the US cyber-attack on Iran in 2019, to name a few. 

The central claim of the book is that there is no set of necessary and sufficient conditions that characterize every instance of an unlawful use of force, but rather these violations should be “characterised by a basket of elements which [need] not be all present and which must be weighed and balanced to determine whether the threshold of the definition is met” (p. 12). In more theoretical terms, Pobjie claims that we should not think of an unlawful use of force as a “concept”, but rather as a “type”. The notion of a “type” is novel and intriguing. Yet, I believe, it warrants further theorization. It is a notion I was unfamiliar with before reading the book, as I suspect will be the case with most international lawyers, and it is introduced rather succinctly -mainly through a comparison between the crime of wilful killing in the Rome Statute (“concept”), and that of Nötigung in the German Criminal Code (“type”) (190-194). A more familiar way to capture this idea I found is through the notion of family resemblances (between instances of unlawful uses of force).

The move from “concept” to “type” proceeds inductively. In chapters 4-6, Pobjie carefully identifies the set of elements or considerations that constitute a prohibited use of force under article 2(4) of the UN Charter and under customary international law. Nevertheless, she observes, in examining different accepted instances of prohibited use of force she identifies “anomalous” cases that don’t necessarily tick every element identified in the previous analysis. For instance, when examining instances of aggression in General Assembly Resolution 3314 -such as military occupation (Art. 3(a)), blockade (Art. 3(c)), and the continuing presence of the armed forces of a state without the consent of the territorial state (Art. 3(e))- she notes that they do not satisfy a standard notion of “force”, because they often “lack physical means and/or (direct) physical effects” (pp. 173-174). Similarly, she uses ICJ’s findings concerning the intentional crossing of a border bearing arms with an intention to use them even before any weapons are fired (Case of the Temple of Preah Vihear) and aerial incursions (Nicaragua) to illustrate other instances of prohibited use of force that neither fulfil the whole range of elements previously identified. Finally, she examines incidents which, although they appear to meet the criteria of an unlawful use of force, have not been treated as such in state practice. These include the shooting down of KAL flight 007 in 1983 by Soviet fighters (p. 176), the shooting down of a Russian fighter by Turkey in 2015, or the invasion of Japanese territorial sea by a Chinese submarine in 2004 (p. 182). Accordingly, these “anomalous” cases (both false negatives and false positives) present a clear challenge to what she contends is the mainstream approach to determining the scope of the rule prohibiting the use of force under international law, which is based on a number of necessary and sufficient conditions. 

The proposed framework, by contrast, is comprised by certain by “fundamental” and “non-essential” requirements (pp. 196-198), whereby the latter can be combined in different ways. Among the former, Pobjie posits that any given unlawful use of force must involves two or more states, affects their international relations, and be conducted against the territorial integrity or political independence of a state, or in a manner incompatible with the purposes of the United Nations. Among the “non-essential” elements, she includes physical force, physical effects, gravity, certain types of targets or objects, and a hostile or coercive intent. Furthermore, insofar they are gradable, these non-essential elements would not only need to be combined, but also weighed in order to determine whether any such incident does indeed qualify as a violation to the prohibition to use force. This proposed framework, Pobjie concludes, fares much better than the standard account in capturing these “anomalous” cases and filtering those which violate this fundamental rule from those which do not. Regardless of any flaws that may still subsist, it is much more accurate and promising than the competing “rule of thumb”/ “legal black box” approach, which, she claims, ultimately depends on the mechanism “I know it when I see it” to identify instances of unlawful use of force (p. 231). 

I find much of the discussion illuminating and thought-provoking. However, I shall hereby concentrate on three aspects of Pobjie’s proposal that, I believe, warrant more careful scrutiny and perhaps greater elaboration. First, in order to assess how Pobjie’s account fares against the mainstream, competing approaches, it is helpful to clarify how any two (or more) such accounts should be compared. In philosophy of science, for instance, it is largely agreed that one theoretical account is more powerful than another when it captures a wider set of phenomena, and when it accounts for the same range of phenomena using a fewer set of considerations and relations. Thus, Leif Wenar suggests, “comprehensiveness and simplicity are relevant dimensions along which … explanations should be measured” (see, e.g., here). Furthermore, there is similarly broad consensus in that rival accounts should be compared in relation to their explanatory power, that is, their capacity to soundly and consistently provide reasons for their ultimate conclusions. Paraphrasing Wenar, we may argue for our purposes that an account of “use of force” should be preferred, when, all else equal, it persuasively and consistently accounts for more cases of use of force, using fewer basic concepts and relations. 

Now, it is possible, in fact quite likely that none of the accounts fare better in all of these dimensions. In such cases, there will be tensions and, ultimately, we may need to make trade-offs. An account may identify more relevant elements but ultimately prove to be unworkable to reach practical conclusions. Alternatively, a simpler theory may be easier to apply but could be less comprehensive, leaving more phenomena “unexplained”. When this type of situation obtains, it is not always clear which dimension we should prioritize in order to identify the most promising account. This difficulty is all the more pressing in the case of legal regimes. This is because being social artifacts, legal rules and principles should not only be assessed on how comprehensive and simple to apply they are (as scientific theories), and how closely and consistently they track our deeper normative commitments (as in ideal theory), but also on the grounds of the different incentives, institutional dynamics, and practical challenges they create. Put differently, in the context of the legal regime on the use of force, what constitutes a better account should also be sensitive to issues of institutionalization, such as the incentives it creates and its broader effects on the reals world. 

The framework put forward in Prohibited Force explicitly stands on its comprehensiveness and explanatory power. It is the best suited, Pobjie argues, both to capture the identified anomalous cases and sort them out on the basis of a complex combination of relevant requirements, carrying different weight in different situations. Nevertheless, this sophisticated “type”-based understanding of prohibited use of force may be too sophisticated to serve its main purpose. Namely, insofar the threshold for violations to the rule prohibiting the use of force requires a complex process of combining different gradable notions and assessing their relative weigh in any particular set of circumstances, this framework is by design incapable of providing a clear (easily accessible) answer to the question at hand. This feature would make disagreement about whether the particular threshold has been met much more likely and intractable. In effect, the more variables involved in a particular judgment the more likely different people will disagree on the precise conclusion of that judgment. 

This feature of Pobjie’s account is particularly problematic in the context of rules regulating the use of (military) force in International Law. As James Morrow has persuasively argued, rules regulating the use of military force are more effective, not when they soundly track normatively right behaviour, but when they adequately shape the strategic expectations of the main relevant actors. In the context of the use of armed force, law has shaped these expectations by setting bright lines that separate permissible from impermissible behaviour. In other words, law in this context must critically have a “noise reducing” function. The less sharp these lines are, the greater the noise; and the greater the noise, Morrow concludes, the greater the possibility of unrestrained violence escalating. This reasoning speaks to the importance of bright lines in construing a legal regime that is effective in influencing behaviour and, in particular, in avoiding the escalation of violence. By contrast, the framework proposed by Pobjie is characterized by blurry lines, and this may ultimately end up undermining a critical pacifying function the jus contra bellum should arguably serve. 

Second, and connectedly, the complex nature of the framework proposed seems to follow Pobjie’s concern for adequately capturing those instances of the use of force at the lowest level of violence. But this push for comprehensiveness may come at a cost, namely, that of lowering the threshold for the violation of this prohibition. Let me explain. Ultimately, any form regulation addressing this kind of legal question will entail having a certain proportion of false positives and false negatives. Taking a stand on the form and scope of any such rule requires deciding on which side we are prepared to err more. This point is standardly acknowledged in certain areas of the law. In the criminal law, for instance, it has been traditionally stated that it is preferable to let ten culpable wrongdoers go free than to put one innocent person in prison. Similarly, the level at which the threshold of the rule prohibiting force under international law is set would have implications as to the consequences and incentives this regulation creates. Stating that a particular incident violated this particular rule is more likely to escalate the reaction, as compared to a statement indicating that a different rule of international law was breached (say, the rule on territorial integrity, non-intervention, and so on).

Prohibited Force does not indicate the level at which the bar should be set, or better, whether it is preferrable to err on the side of false positives or false negatives. Pobjie’s analysis of this issue is concerned with the “size of the gap between prohibited force under article 2(4) and an armed attack giving rise to a right of self-defense under article 51” (p. 7). Ultimately, she does not take a position on how wide this gap is or should be. But the book transpires a concern for identifying instances of prohibited use of force at the lowest possible end of the spectrum, to the exclusion of other legal rules that any of these incidents may breach. As Pobjie herself acknowledges, International Law has several other rules that may better capture some of these incidents, including the rule prohibiting intervention in the internal affairs of other states, the rules on territorial integrity banning extraterritorial enforcement action, international human rights law (and its extraterritorial application, as required), among others. By contrast, the finding of a violation of the prohibition to use force seems much more likely to create the expectation of a harsher response, than the violation of any other international rule. When all we see is nails, we are likely to only use a hammer. 

In sum, I suggest that the complex framework advocated in Prohibited Force is both perhaps too convoluted (sophisticated), and not sensitive enough to the perils of a wide understanding of the scope of the prohibition to use force under international law. I have claimed that these flaws are not related to how accurate or persuasive this novel framework ultimately is, but rather of the type of incentives it creates and consequences it may be expected to have in the real world. These observations may push us to re-examine the extent to which the simpler, mainstream approach is inadequate to sort the “anomalous” cases. To put it perhaps too quickly, it may be that some of the cases lacking actual physical force or physical effects, could be captured by the prohibition of the threat to use force, which is similarly provided for in article 2(4) of the UN Charter and under customary international law. And yet this is a route Pobjie explicitly excludes from her inquiry. Similarly, one could question whether some of the false-positive cases (those which comply with the formal requirements of the mainstream concept of the use of force and yet were not considered violations of the rule) cannot be sorted out by reference to a minimum standard of gravity, or a more controversial requirement of hostile intent directed against a foreign state. Although I cannot explore here the extent to which any of these two moves can ultimately ease Pobjie’s dissatisfaction with the mainstream approach, I believe there are plausible reasons not to be too quick in abandoning the conceptual approach to the use of force.

Let me conclude by restating the important. The book is a must read for anyone working in the area. Erin Pobjie’s writing is clear and engaging. The book is carefully researched, thought-provoking, and deeply innovative. In this short post I have simply sought to press Pobjie on some of the institutional implications her argument may generate, in the hope of moving the conversation forward.

The author is grateful to Larissa van den Herik for an illuminating conversation on the topic of this post while preparing these comments.